Natural killer (NK) cells are innate lymphocytes that live up to their name, capable of killing infected or tumor cells within minutes of exposure. Unlik their T and B cell counterparts in the adaptive immune system, harnessing NK cells for vaccination has not been a major focus in vaccine development. However, several recent findings from the co-PIs of this application indicate that these cells deserve more attention. In animal models, NK cells can remember HIV antigens, responding faster and better the second time. Even more importantly, this application presents exciting new data that human NK cells have the same properties and are capable of potent recall responses to HIV antigens. Our studies have also revealed an unanticipated diversity of the human NK cell repertoire, demonstrating that more than 100,000 subtypes exist, overturning the paradigm of NK cell homogeneity. This diversity matters: we recently discovered that high NK repertoire diversity is associated with significantly increased risk of HIV-1 acquisition in vivo. Thus, NK cells share two of the most important characteristics of the adaptive immune response - memory and diversity. Yet, they can respond to threats within minutes rather than days, differentiating them from slower-acting classic adaptive responses. These features make NK cells uniquely poised to prevent and control HIV. However, a lack of understanding of how to optimally harness NK cells while minimizing off-target effects remains a major hurdle to developing NK cell-based therapies. To overcome this hurdle, we propose the first comprehensive analysis of how antigen exposure shapes human NK cell responses to HIV. We will use our established and highly sensitive assays, human donor derived immune cells, and humanized mice, to identify the mechanisms by which antigen exposure shapes the repertoire and drives potent NK cell responses to HIV. These studies will provide the rationale to develop novel vaccines that exploit the antiviral activity of NK cells to protect humans from HIV infection.